1. Field of the Invention
The present invention relates generally to a hydraulic brake booster for a vehicle, and more particularly, to a hydraulic brake booster for use with an open center type power steering system.
2. Description of the Prior Art
Conventionally, many hydraulic brake boosters have been proposed. For example, within a brake booster having a pressure difference valve means, the fluid pressure being circulated by a power steering pump must be high enough, under the control of the pressure difference valve means, in order to improve the response of the brake operation when the brakes are to be applied suddenly in an emergency. A circulated fluid pressure which is too high, however, imposes an extra load upon the pump even when the brakes are not applied.
Hydraulic brake boosters having accumulators to accumulate the fluid pressure for the brakes in case of an emergency are also conventionally arranged such that the fluid pressure which is generated upon the operation of the brake booster or the power steering gear can be accumulated within the accumulators. It is noted here however, that the conventional accumulators have been of the gas and spring type, and the fluid pressure generated upon the operation of the booster or the power steering gear must overcome the pressure for charging the gas within the accumulators of the gas type or the setting pressure of the spring within the accumulators of the spring type in order to in fact be accumulated within the accumulators.
Yet, sometimes the fluid pressure generated upon the operation of the booster or the power steering gear has not been high enough. In other words the generated fluid pressure has been less than the pressure required for charging the gas or for setting the pressure of the spring, and thus, the operations of the accumulation often have been incomplete.